The present invention relates generally to satellite communication systems. More specifically, the present invention relates to controlling uplink power in a satellite communication system.
Satellites have long been used to provide communication services to large regions of the earth. Modern satellites have been proposed for operation at frequencies of 20 to 30 GHz and to produce a beam which may cover an area on the earth (cell) with a diameter of 300 to 400 miles. Many of these cells are combined to cover large geographical areas. For bandwidth efficiency, non-adjacent cells often communicate using identical frequencies. Frequency re-use between non-adjacent cells introduces co-channel interference into the system. For example, imperfect antenna patterns often contain significant side lobes. Signals transmitted in one cell with large side lobes interfere with signals transmitted in a non-adjacent cell at the same frequencies. As co-channel interference increases, the system experiences a lower signal-to-background ratio (SBR), and hence, a higher error rate. Given the desire to minimize the error rate, or maximize the SBR, one goal in a cellular satellite communication system is to minimize the contribution to background level (B) caused by co-channel interference. Co-channel interference (CCI) is, in part, dependent upon the power level at which signals are transmitted. Hence, co-channel interference may be reduced by reducing the power used to transmit signals.
Several factors adversely impact signal integrity in satellite communication systems. These factors, for example, include antenna gain within and across a cell, antenna beam pointing error, atmospheric conditions and co-channel interference from other signals. These factors typically lower the SBR of the signal, resulting in a higher error rate. One way to overcome the adverse effects of these factors is to increase the SBR by increasing the signal strength (S), and thus increasing the power used to transmit signals.
The desire to minimize co-channel interference by minimizing the transmit power and the desire to maximize signal strength S by maximizing the transmit power represent conflicting goals. Satellite systems employing both techniques to maximize the SBR must maintain a balance between too much power, resulting in a high co-channel interference level, and too little power, resulting in a low signal strength (S).
Ideally, the uplink power level threshold, to which a user earth terminal (xe2x80x9cUETxe2x80x9d) initially converges, is very close to the power level used during communication. However changing atmospheric conditions, changing antenna beam characteristics, and changing noise environments may result in a substantial disparity between the uplink power level to which the earth terminal converges at the outset of communication, and the uplink power level, to which the earth terminal converges during the course of communication. The power level disparity, in turn, may result in inefficient bandwidth utilization. A substantial amount of bandwidth may be wasted by having an earth terminal converge its uplink transmit power to an uplink power level threshold that is no longer adequate due to a changing interference environment. A substantial amount of data may be lost before the UET adjusts to a more adequate uplink power level.
U.S. Pat. No. 4,910,792, entitled xe2x80x9cUp-link Power Control in Satellite Communications Systemxe2x80x9d (the ""792 patent) illustrates one approach for controlling uplink transmission power to compensate for rain attenuation. The ""792 patent illustrates a system including a-number of user stations 59, a reference earth station 58, and a satellite 50, identified at column 1, lines 41-43, which is xe2x80x9ca mere repeater of signals, but has no facility to measure the power transmitted from each earth station.xe2x80x9d In operation, the transmission power of a reference signal transmitted from the reference earth station 58 is adjusted so that the received reference signal at the satellite is constant. Each user station 59 transmits a signal which is relayed to the satellite and back to the user station 59. Each of the earth stations 59 then detects the difference between the received reference signal from the reference earth station through the satellite and the level of the received signal with was sent from itself and relayed by the satellite. Each of the earth stations 59 then adjusts its uplink power based on the difference between the signals. That is, the ""792 patent assumes that the reference burst 60 from the reference station 59 is received by the user station 59 with attenuation only on the downlink, while the burst 61 sent from the user station 59 is received at the user station 59 with the attenuation on both the uplink and downlink. Therefore, the difference between the received reference burst signal 60, and the user station burst 61 sent from the user station itself is the attenuation 62 in the uplink, as shown in FIG. 4(b). The ""792 patent applies only to systems employing bent-pipe transponders, which are not present in a processing satellite communication system.
U.S. Pat. No. 5,864,547, entitled xe2x80x9cMethod and System for Controlling Uplirnk Power in a High Data Rate Satellite Communication System Employing On-Board Demodulation and Remodulationxe2x80x9d (the ""547 patent) illustrates another approach for controlling uplink transmission power. In operation, as shown in FIGS. 1 and 5, a downlink error rate of the data in a downlink data stream is determined based on known data bits transmitted by a satellite and received by a receiving terminal 12. An end-to-end error rate of the uplink data stream and the downlink data stream is then determined based on the number of errors in received data transmitted by a first user terminal 11 to the receiving terminal 12. The error rate of the uplink is then indirectly estimated based on the downlink error rate and the end-to-end error rate with reference to a lookup table. Finally, the power of the uplink is controlled based on the indirect estimate of error rate of the uplink. Thus, the ""547 patent relies on an indirect estimate of uplink signal quality using downlink signals. Therefore, errors introduced in the downlink may not reliably be separable from errors introduced in the uplink. The ""547 patent does not determine the uplink error rate directly.
Thus, a need has long existed for a system and method for controlling the uplink power in a satellite communication system. A need has especially existed for such a system and method able to control uplink power in an uplink channel affected by gain roll-off, antenna pointing errors, atmospheric attenuation, and co-channel interference. A need has also long existed for a system and method for controlling the uplink power level to compensate for a dynamic interference environment.
The preferred embodiments of the present invention provide a method and apparatus for power threshold leveling in a satellite communication system. A preferred embodiment of the present invention first obtains uplink power level information for a plurality of earth terminals. The uplink power level information may, for example, include indications of uplink reference power levels, uplink power levels converged to during communication, or the difference between the two. The uplink power level information may also include frequency channel, time slot, and coding level information. Statistical characteristics are then determined for the uplink power level information received from the plurality of earth terminals. For example, the mean or variance of the uplink power levels may be determined. An uplink power level threshold adjustment is determined from the statistical characteristics and used to modify the uplink power level threshold. The apparatus of the preferred embodiment includes a satellite communication system including a satellite, user earth terminals (UETs), and a network control center (NCC). The satellite stores uplink power level threshold information which the satellite uses initially to control the uplink power level of UETs. The satellite intermittently receives uplink power level threshold adjustments from the NCC and uses these adjustments to modify the stored uplink power level threshold information. The UETs receive power profile information requests from the NCC and respond by sending uplink power profile information back to the NCC. The uplink power profile information may include a list of frequency channels and time slots in use by the UET along with respective uplink power level and coding information. The NCC obtains power level information for the UETs. The power level information includes uplink power level information for the UETs and may also include frequency channel, time slot, and coding information. The power level information may be obtained from various system entities, like the UETs, at various points in time. The NCC statistically analyzes the uplink power level information to arrive at an uplink power threshold adjustment, which the NCC communicates to the satellite. The NCC may also determine uplink power offsets corresponding to different coding levels used by the system, which the NCC then distributes to the UETs or other system entities.
The preferred embodiment of the present invention may be a component of a comprehensive uplink power control system, such as that described in U.S. patent application Ser. No. 09/596,683 filed Jun. 19, 2000, titled xe2x80x9cComprehensive System and Method for Uplink Power Control in a Satellite Communication System.xe2x80x9d